1. Field of the Invention
The present invention relates to a signal conversion apparatus and to a signal conversion method for protecting the copyright of information signals after signal conversion when, for example, an information signal, such as a video signal or an audio signal, is converted and output.
2. Description of the Related Art
Various software has come to be distributed through various types of recording media, such as video tape or DVD (digital video disk), the Internet, and broadcasting media. However, there is a problem in that software which is distributed on a large scale in this manner might be copied without limit, and various copy-prevention measures have been taken conventionally.
For example, for analog video signals, although it is not a method for directly prohibiting copying of the analog video signals, there is a method for effectively preventing copying by using a difference in methods of AGC (automatic gain control) or a difference in characteristics of APC (automatic phase control) between, for example, a VTR as a recording apparatus and a monitor receiver for providing a video.
A method that uses a difference in methods of AGC, for example, in a VTR, AGC is performed in accordance with a pseudo-synchronization signal inserted into a video signal, and in a monitor receiver, an AGC method which is not dependent on this pseudo-synchronization signal is adopted, is an example of the former case. When an analog video signal is recorded on an original recording medium, a pseudo-synchronization signal whose level is extremely high as a synchronization signal for AGC is inserted in advance, and this pseudo-synchronization signal whose level is extremely high is inserted, as a synchronization signal for AGC, into a video signal to be supplied from a playback VTR to a recording VTR.
Also, a method that uses a difference in the characteristics of the APC, for example, APC in a VTR follows a color burst signal in a video signal at a short time constant, and APC in a monitor receiver follows a color burst signal at a relatively long time constant, is an example of the latter case. When an analog video signal is recorded on an original recording medium, the phase of a color burst signal of a video signal is partially inverted in advance, and a video signal in which the phase of a color burst signal is partially inverted is output as a video signal to be supplied from a playback VTR to a recording VTR.
As a result of the above, in a monitor receiver which receives an analog video signal from a playback VTR, video is played back normally without being influenced by the pseudo-synchronization signal or the partial inversion of the phase of the color burst signal used for APC.
However, in a VTR which receives an analog video signal, to which a pseudo-synchronization signal is inserted from a playback VTR as described above or which is subjected to phase inversion control of a color burst signal, and which records this signal on a recording medium, gain control or phase control based on an input signal cannot be performed normally, and the video signal cannot be recorded normally. Therefore, even if the recorded video signal is played back, it is possible to make a normal video which can be viewed or listened to not to be played back.
When an analog video signal is handled in this manner, rather than prohibiting copying, the purpose is to produce a playback video which cannot be viewed or listened to normally, and this is a kind of passive copying prevention control.
In contrast, when digitized information, for example, a digital video signal, is handled, copying control information indicating copying prohibition or copy permission is added, as digital data, to the video signal, and is recorded on a recording medium or is transmitted, so that direct copying prevention control, such as prohibiting copying, is performed in accordance with the copying control information added to the video signal.
For example, when main digital information recorded on a recording medium having copying control information added thereto is copied to another recording medium, in a digital playback apparatus, information is read from the recording medium, the copying control information is obtained together with the main digital information, and this information is sent to a digital recording apparatus through a digital transmission path.
Then, in the digital recording apparatus, a copying control signal is detected from the information received through the digital transmission path, and the control content thereof is determined. Then, recording control of the main digital information is performed based on the determination result.
Therefore, when the determined content of the copying control indicates that the recording of the main digital information input through the digital transmission path is allowed, the input main digital information is converted into digital information suitable for recording and is written into another recording medium, thus performing recording. When, on the other hand, the determined content of the copying control indicates copy prohibition, a process for recording the input main digital information is not performed.
Also, in addition to main information being copy-prohibited or copy-permitted regardless of whether it is an analog signal or a digital signal, a copy control method of generation limitation has been conceived such that copying is permitted once, but copying from the copy is prohibited. As a copy control information method which is capable of performing this copy generation management, a CGMS (Copy Generation Management System) method and a method that uses a digital watermark process have been proposed.
A digital watermark process is a process for embedding information as noise into a portion (which is not important for human perception) present in a video signal and an audio signal, that is, a portion which is not redundant with respect to music and video. Additional information embedded into a video signal and an audio signal by such a digital watermark process is not easily removed from the video signal and the audio signal. On the other hand, even after a filtering process is performed on a video signal and an audio signal (in the case of digital information, even after a data compression process is performed thereon), it is possible to detect the additional information embedded therein from the video signal and the audio signal.
In the case of a copying control method which uses this digital watermark process, the following four states are made to represent the copy generation and the copy limitation state of a video signal and an audio signal to which the relevant digital watermark information is superposed according to the additional information to be embedded:
{circle around (1)} “Copying is possible (Copy Free)”
{circle around (2)} “Copying is possible once (Copying is possible for one generation) (One Copy)”
{circle around (3)} “Subsequent copying is prohibited (No More Copy)”
{circle around (4)} “Copying is absolutely prohibited (Never Copy)”
“{circle around (1)} “Copying is possible (Copy Free)”” indicates that free copying of audio signals and video signals is possible. “{circle around (2)} “Copying is possible once (copying is possible for one generation)”” indicates that copying of audio signals and video signals is possible once. “{circle around (3)} “Subsequent copying is prohibited (No More Copy)”” indicates that the relevant audio signals and video signals are copied from the audio signals and video signals in a state of {circle around (2)} in which copying is possible once, and subsequent copying is prohibited. “{circle around (4)} “Copying is absolutely prohibited (Never Copy)”” indicates that copying is absolutely prohibited.
When the digital watermark information superposed onto the video signals and audio signals is “Copying is possible once (One Copy)”, a recording apparatus which supports a digital watermark process (that is, supports a copy limitation process) determines that copy recording of the video signals and audio signals is possible and performs recording, and digital watermark information replaced with the “Subsequent copying is prohibited (No More Copy)” is superposed onto the recorded video signals and audio signals. When the digital watermark information superposed onto the video signals and the audio signals to be recorded is “Subsequent copying is prohibited (No More Copy)”, a recording apparatus which supports a digital watermark process determines that the copy recording of the video signals and audio signals is prohibited, and prohibits the performing of the recording.
The CGMS method is a method in which, in the case of, for example, an analog video signal, 2-bit additional information for copy control is superposed (this case is particularly called “CGMS-A”) in one specific horizontal interval within a vertical blanking period of the analog video signal, and in the case of a digital video signal, 2-bit additional information for copy control is added (this case is particularly called “CGMS-D”) to digital image data and is transmitted.
In the case of this CGMS method, the meaning of the 2-bit information (hereinafter referred to as “CGMS information”) is as follows:
[00] . . . Copying is possible
[10] . . . Copying is possible once (copying is possible for one generation)
[11] . . . Copying is prohibited (Copying is absolutely prohibited)
In the CGMS method, there is no state for the above-mentioned “Subsequent copying is prohibited”.
When the CGMS information added to the video signal is [10], a CGMS-compatible recording apparatus determines that copy recording of the video signals is possible and performs recording, and the CGMS information which is rewritten into [11] is added to the recorded video signals. Then, when the CGMS information added to the video signal to be recorded is [11], the CGMS-compatible recording apparatus determines that copy recording of the video signals is prohibited, and performing of recording is prohibited.
As described above, in order to protect the copyright of content information, such as video signals and audio signals, various copy prevention measures have been considered, and by using one or more of these measures, protection of the copyright of the content information can be ensured.
However, even in the case of main information in which a copy prevention measure such as that described above is taken, there is a case in which the copyright of the main information cannot be appropriately protected, for example, as a result of this information being subjected to signal conversion, information added to the main information signal for the purpose of the protection of copyright is lost or deteriorated, making it possible to copy main information, the copying of which is prohibited.
When, for example, an analog video signal, to which CGMS information, etc., is added in a vertical blanking period, recorded on a recording medium is read and this signal is converted into a digital video signal and output, the additional information, such as the CGMS information, added in the vertical blanking period, disappears. Therefore, in this case, even if CGMS information for prohibiting copying has been added to the original analog video signal, the digital video signal after signal conversion can be copied.
In addition, when mutual signal conversion between a high-definition television signal (hereinafter abbreviated as “HDTV”) and a standard television signal (hereinafter abbreviated as “SDTV”), conversion of the number of scanning lines of a video signal, or mutual signal conversion between progressive scanning and interlaced scanning is performed, additional information added to a video signal before conversion may deteriorate in the video signal after conversion or the state of the additional information may change.
Even in such a case, as a result of the additional information added to the video signal after conversion not being accurately detected, copying of the signal after conversion may become possible despite the fact that copying of the signal before conversion is prohibited.
As described above, an apparatus for performing signal conversion becomes what is commonly called a bypass apparatus for making possible recording of a signal whose copy has been prohibited, making it impossible to appropriately protect the copyright of the content information whose copying is prohibited or limited.
For this reason, in an apparatus for performing signal conversion, there is a need to newly mount an apparatus for adding additional information used to protect the copyright to a signal after signal conversion. However, this increases the cost of the apparatus for performing signal conversion. Also, when the signal after conversion is an analog audio signal or HDTV, since the copyright protection technology for these signals has not yet been established completely, it is not possible to reliably protect the copyright of the analog audio signal and HDTV.